For SPECT imaging with attenuation correction (AC), attenuation maps are usually obtained using one of the following techniques:
(A) Isotopic transmission sources may be used with the same conventional Anger cameras that are also used for emission acquisition, or
(B) CT images that are obtained using a separate set of detectors for X-ray detection.
When isotopic transmission sources are used, simultaneous emission/transmission scans or interleaved transmission/emission scans can be used to create perfect emission/transmission registration. However, these transmission scans can take a long time as compared to emission scans.
CT image scans in approach (B) allows very fast CT scans and high quality attenuation maps converted from the CT images. However, the CT scans require the use of a separate detector set. Also, the transmission and emission scans are sequential. In addition, fast CT scans are also subjected to respiratory motion related misregistration with emission scans and CT scans are in general associated with large patient doses.